1. Field of the Invention
The present invention relates to high density memory devices based on phase change based memory materials, and to methods for programming such devices.
2. Description of Related Art
Phase change based memory materials have been used in nonvolatile random access memory cells. Phase change materials, such as chalcogenides, can be caused to change between an amorphous state and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher resistivity than the generally crystalline state, which can be readily sensed to indicate data.
Phase change materials are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase, and a second structural state in which the material is in a generally crystalline solid phase in the active region of the cell. The term amorphous is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has detectable characteristics such as higher electrical resistivity than the crystalline phase. The term crystalline is used to refer to a relatively more ordered structure, more ordered than in an amorphous structure, which has detectable characteristics such as lower electrical resistivity than the amorphous phase. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material may be switched into either different solid phases or mixtures of the two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states.
The change from the amorphous to the crystalline state, referred to as set or program herein, is generally a lower current operation, requiring a current that is sufficient to raise the phase change material to a level between a phase transition temperature and a melting temperature. The change from crystalline to amorphous, referred to as reset or erase herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process, allowing at least a portion of the phase change material to stabilize in the amorphous state.
The amount of amorphous phase material in a reset state will vary among memory cells in an array because of variations in materials, manufacturing processes, and the operating environment. These variations result in different programming characteristics including differences in the amount of energy required to program the memory cells.
Thus, applying the same programming pulse to each memory cell in an array will result in a wide distribution of resistance values. Additionally, due to these variations some cells may be set to a resistance outside the resistance range corresponding to the programmed data value, resulting in errors in the stored data of the memory cell.
Thus, there is a need for better ways to program memory cells in memory systems using phase change materials.